1. Field of the Invention
The present invention relates to a display device. More particularly, the present invention relates to a thin film transistor applied to a display device.
2. Discussion of the Related Art
A thin film transistor (TFT) is used as a switching device or a driving device, wherein the switching device controls an operation of each pixel in a display device such as a liquid crystal display (LCD) or an organic light emitting device (OLED), and the driving device drives each pixel.
The thin film transistor includes a gate electrode, an active layer, and source/drain electrodes, and may be categorized into a staggered structure and a coplanar structure based on the arrangement of the electrodes.
In the staggered structure, the gate electrode and the source/drain electrodes are arranged up and down on the basis of the active layer, and in the coplanar structure, the gate electrode and the source/drain electrodes are arranged on the same plane.
The thin film transistor of the staggered structure may be divided into a back channel etched (BCE) type and an etch stopper layer (ESL) type depending on a method for forming a channel. Since the etch stopper layer type thin film transistor has an advantage in that an etch stopper layer is formed on the active layer to prevent the active layer from being over-etched, its use has been increased.
FIGS. 1A to 1E are cross-sectional views illustrating a related art process steps of manufacturing an ESL type thin film transistor substrate.
First of all, as shown in FIG. 1A, a gate electrode 20 is formed on a substrate 10, and a gate insulating film 25 is formed on an entire surface of the substrate including the gate electrode 20.
Next, as shown in FIG. 1B, after an active layer 30a and an etch stopper layer 40a are sequentially deposited on the gate insulating film 25, the etch stopper layer 40a is patterned to form a predetermined etch stopper 40 as shown in FIG. 1C. The etch stopper 40 serves as a stopper during a later etching process.
Next, as shown in FIG. 1D, an ohmic contact layer 50a and a source/drain electrode layer 60a are sequentially deposited on the entire surface of the substrate including the etch stopper 40.
Next, as illustrated in FIG. 1E, the source/drain electrode 60a is patterned to form a source electrode 62 and a drain electrode 64, and the ohmic contact layer 50a and the active layer 30a are etched using the source and drain electrodes 62 and 64 as masks, whereby an ohmic contact layer 50 and an active layer 30 are formed at a predetermined pattern.
However, since the aforementioned thin film transistor according to the related art, as shown in FIGS. 1A to 1E, has a single gate structure having one gate electrode 20, it is difficult to obtain output saturation characteristics. In addition, there is a non-negligible gap between transfer curves according to voltages between source and drain of a thin film transistor within a subthreshold region, whereby problems of crosstalk or non-uniform luminance such as spots may occur on a screen. Especially, if the thin film transistor with the single gate electrode structure is applied to the organic light emitting device, a compensation capacity may be deteriorated.
Also, in case of the related art thin film transistor with the etch stopper 40, the thin film transistor is inevitably increased in size due to an overlay rule in between each layer. Due to the increased size of thin film transistor, an overlap area between the gate electrode 20 and the source/drain electrodes 62/64 is increased in size, to thereby increase a capacitance of the thin film transistor.